Recovery of bitumen from mined oilsands involves separation of a bitumen-rich froth from an oilsand slurry, and treatment of the froth to remove impurities and produce a marketable product. Known paraffinic froth treatment processes involve the addition of a paraffinic solvent to the bitumen-rich froth which enables separation of the mineral and water contaminants and production of a marketable bitumen product. Asphaltenes in the bitumen precipitate if sufficient paraffinic solvent is mixed with froth. In a froth settler vessel the precipitated asphaltenes settle along with the mineral and water contaminants. This gravity settling process separates the diluted froth into two streams; a diluted bitumen product at a top and an underflow at a bottom. The underflow is a mixture of solids, water, bitumen, solvent and precipitated asphaltenes.
Typically, the paraffinic froth treatment process can be sub-divided into three components: froth settler/separation, solvent recovery and tailings solvent recovery. After the froth settling/separation, the diluted bitumen product is sent to a solvent recovery unit (SRU) where solvent is recovered for re-use and clean bitumen is obtained as the feed to an upgrading process. Underflow from froth settling/separation is sent to a tailing solvent recovery unit (TSRU) to recover residual solvent.
Conventional settler/separation operations implement a counter-current, multi-stage system to produce two streams, a solvent-rich diluted bitumen product (which includes some asphaltenes), and an underflow tailings stream containing solids, water, bitumen, solvent and precipitated asphaltenes.
In a solvent recovery unit (SRU), a distillation column is employed for recovering relatively volatile solvent from the solvent-rich diluted bitumen product and delivering a clean, solvent-free, bitumen product and solvent stream for reuse.
In a tailings solvent recovery unit (TSRU), residual solvent in the settler underflow stream is recovered to ensure that a final tailings stream contains minimal amounts of residual solvent. Conventionally, a TSRU vessel is a flash tower into which the hot, pressurized tailings stream is released, resulting in rapid vaporization of the bulk of the residual solvent from the tailings. Steam may be added to further scavenge solvent from the tailings.
Two prior art processes which implement counter-current processes are Canadian Patent application 2,454,942 to True North Energy and Canadian patent 2,521,248 to Shell et al.
CA 2,454,942 describes a process in which a low-molecular weight paraffin is used as the solvent in a two-stage settler configuration. Underflow from a first stage settler is directed as influent to a second stage settler. In this process, the only introduction of solvent is by addition to the influent of the second stage settler, and the second stage settler overflow is recycled to the influent of the first stage settler. The overflow product of the first stage settler is the final product to the SRU. Accordingly, solvent is only added to the first stage settler by recycling back from the second stage settler overflow. The process is known as a counter-current process as the solvent flows counter-currently to the flow of froth. The second stage underflow is the influent to the TSRU.
CA 2,521,248 describes a process which applies three settlers. As in the True North process, there is a recycling of the overflow from the second stage settler to the influent of the first stage settler. The second stage overflow contains mainly solvent and diluted bitumen. Further, the overflow of the third stage settler is recycled back to the influent of the second stage settler. The overflow product of the first stage settler is the final product to the SRU. The only introduction of solvent is by addition to the influent of the third stage settler, and therefore solvent is only recycled to the first and second stage settlers by recycling back from the third stage settler. Again, a counter-current process is described with the overflow product of the first stage settler being the final diluted bitumen product to the SRU and the third stage underflow being the influent to the TSRU.
Other than the diluted bitumen product stream, each of the True North and Shell processes result in a single, high water content waste stream for downstream processing in the TSRU. The True North process results in about 7% of the bitumen in the froth being precipitated as asphaltenes and lost to the tailings. Asphaltenes are not a desirable component in a tailings stream or the bitumen product stream.
There is a need for recovering at least some of the asphaltenes otherwise lost to the tailings for reducing environmental impact, or for other beneficial purposes. Further, improved handling of the asphaltenes fraction can result in reduced asphaltene content in the diluted bitumen product. Simply, there are synergistic objectives to reduce hydrocarbon loss in tailings and to recover a valuable product from what is otherwise wasted.